1. Field of the Invention
This invention relates to gas detectors. More specifically, the present invention is directed to gas detectors for detecting very small gas concentrations in a gas mixture.
2. Description of the Prior Art
The detection of very small concentrations of gases or vapors in air or other gas mixtures by using an ionization measuring cell is known in principle from U.S. Pat. No. 4,119,851. In that well-known method of ion-mobility spectroscopy, ions move under the influence of an electrostatic field through a drift zone. A voltage in the order of 2 kV is provided between the radiation source and a collector electrode for generating this eletrical field. Since light ions move faster than heavy ions, the ions started at a predetermined time reach the collector electrode after different times-of-flight depending on their respective molecular weight. Short current pulses are generated when such an ion package reaches the collector electrode. The time position of such a current pulse within a time-of-flight spectrum depends on the mass of the respective ions and therewith permits sorting the ions dependent on their mass. The intensity of the current pulse is a measure for the number of ions of a predetermined mass within the ion package which started at a predetermined time. For detecting particular ions therefor a time-of-flight analysis of the ion current is required. Furthermore, the drift zone must have a sufficient length in order that the time distance between the arrival of ions of different mass at the collector electrode permits a separation of the individual current pulses and thereby permits the detection cf individual groups of ions. Such a prior art device requires a high voltage supply, a time-of-flight analysis and a relatively long drift zone which significantly increases the cost and size of the device. Accordingly, it would be desirable to provide a method and apparatus for detecting gas concentrations which avoids those limitations of the prior art to produce a compact, easily mobile, reliable and relatively inexpensive device.